This invention relates to a device for providing a signal which is representative of enthalpy and is particularly directed to a device for combining temperature and relative humidity to produce this signal representative of enthalpy.
In the control of air conditioning and heating systems, it has long been desired to use enthalpy, particularly in those systems which must choose the portion of return air and the portion of outdoor air to be treated and then supplied to the zones under the control of the system. For example, during the air conditioning or summer season, it is desirable to bring in a large portion of outdoor air when the outdoor air is cool and dry. If the outdoor air is cool but humid, it may be desirable to increase the portion of return air mixed with the outdoor air to reduce the amount of dehumidification necessary to provide comfortable conditions. Similarly, if the outdoor air is warm but relatively dry, it still may be advantageous to increase the portion of return air mixed with the outdoor air to maintain at a minimum the amount of energy expended to cool this air to within the comfortable range. And, of course, if the outdoor air is both warm and humid, the amount of energy needed to cool and dehumidify this air is substantial and, therefore, the portion of return air is increased and the portion of outdoor air is decreased.
Various prior art arrangements have been used to provide this type of control. Some of these arrangements measure only outdoor air temperature and return air temperature which merely gives an indication of the sensible heat content of the air that is being measured. Total heat (specific heat content plus latent heat content), however, is a better measurement of how much temperature and humidity treatment is necessary to provide comfortable conditions since specific heat content measurement along ignores the moisture content of the air being controlled. It is therefore necessary to measure the latent heat content of the air as well.
In order to measure the total heat content (i.e. latent heat plus sensible heat) of air, prior art arrangements measured both the temperature and humidity of the outdoor air as well as the temperature and humidity of the indoor air. Such a system, however, must rely upon four sensors for providing this measurement of total heat, i.e. enthalpy. The use of humidity and temperature sensors have been in very complex circuit arrangements in order to give a reasonable approximation of total heat or they have been used together with computer programs for calculating enthalpy according to the prescribed enthalpy formula. In order to reduce the number of sensors required to measure this total heat or enthalpy, the prior art systems utilized wet bulb temperature sensors which, if one reviews the psychometric chart, gives a fairly accurate indication of total heat or enthalpy. However, wet bulb temperature sensors are complex and require a constant source of water. Applicant has devised a system for giving a reasonably accurate approximation of enthalpy relying upon a simple circuit having a humidity input and a temperature input.